hot tub

[ELZ4742]

have you tested the feed? had an issue similar with a shower, worked normally on the cold with the light on but tripped once you turned the element on.
turned out that the cable had a been nicked with a clip.

 

[jj4091]

You seem to have proved it is not the element itself so as I said before the fault would seem to be in the wiring to it, as it did not trip when disconnected I would suspect a neutral to earth short cct.

 

[paz7330]

It is an armoured cable from the garage, so would be difficult to nick, seems to me that a good electrician will now be able to sort this out, I am fine at paying for an electrician, just don't want to call out someone from the hottub company